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SAN DIEGO — A woman fell out of a moving party bus on the freeway in the South Bay, hitting her head and suffering “major life-threatening injuries,” officials said.

The accident happened around 8 p.m. Saturday on Interstate 5 in National City, just south of Civic Center Drive, according to California Highway Patrol.

Investigators were interviewing other people on the bus for further details, but officers were initially told the woman was making her way to the vehicle’s restroom when she somehow opened an emergency door, tumbling down into the freeway lanes, CHP said.

The bus pulled over and partygoers rushed to help the woman along with other passersby who saw her fall. Video from OnScene.TV showed she had a serious head injury. Paramedics and officers arrived and the woman was loaded onto an ambulance and taken to the hospital.

The woman’s injuries were said to be grave, but further details or an update on her status were not immediately available Sunday morning.